Wireless communication systems are an integral component of the ongoing technology revolution. Mobile radio communication systems, such as cellular telephone systems that provide mobile communications for wide areas of coverage, are evolving at an exponential rate. The wireless networks have become so popular that they essentially replace the traditional wired networks for users in large areas.
Wireless systems can be classified according to the method used to provide access to multiple users seeking to utilize the system in parallel, such as Time Division Multiple Access System (TDMA), Code Division Multiple Access (CDMA), etc. Generally, CDMA is a type of modulation also known as Direct Sequence Spread Spectrum (DSSS). In a DSSS system, channels are defined by complementary, orthogonal or pseudo-random spreading sequences or codes, where each user is assigned a unique spreading sequence. The spreading sequence has a frequency much higher than that of a user's information signal. DSSS signals have spectral characteristics of bandwidth limited white noise in the radio frequency (RF) spectrum. Typically, such a DSSS signal is likely to have one or more interference signals present. The task of identifying interference in a DSSS signal represents a classic detection-of-signals-in-noise problem, where the “noise” that needs to be detected is in fact a signal in a spectrum whose characteristics are similar to white noise. In other words, the white noise is the signal that needs to be preserved, and the narrow band interference signal is undesired.
FIG. 1 illustrates an exemplary telecommunication system 10, which may include mobile units 12, 13A, 13B, 13C, and 13D, a number of base stations, two of which are shown in FIG. 1 at reference numerals 14 and 16, and a switching station 18 to which each of the base stations 14, 16 may be interfaced. The base stations 14, 16 and the switching station 18 may be collectively referred to as network infrastructure.
During operation, the mobile units 12, 13A, 13B, 13C, and 13D exchange voice, data or other information with one of the base stations 14, 16, each of which is connected to a conventional land line telephone network. For example, information, such as voice information, transferred from the mobile unit 12 to one of the base stations 14, 16 is coupled from the base station to the telephone network to thereby connect the mobile unit 12 with a land line telephone so that the land line telephone may receive the voice information. Conversely, information such as voice information, may be transferred from a land line telephone to one of the base stations 14, 16, wherein the base station in turn transfers the information to the mobile unit 12.
The mobile units 12, 13A, 13B, 13C, and 13D and the base stations 14, 16 may exchange information in either analog or digital format. For the purposes of this description, it is assumed that the mobile unit 12 is a narrowband digital unit and that the mobile units 13A, 13B, 13C, and 13D are wideband digital units. Additionally, it is assumed that the base station 14 is a narrowband digital base station that communicates with the mobile unit 12 and that the base station 16 is a wideband digital base station that communicates with the mobile units 13A, 13B, 13C, and 13D using DSSS signals.
Digital format communication may take place using, for example, narrowband 200 kilohertz (kHz) channels. The Groupe Spécial Mobile (GSM) system is one example of a digital communication system in which the mobile unit 12 communicates with the base station 14 using narrowband channels. The mobile units 13A, 13B, 13C, and 13D communicate with the base stations 16 using a form of DSSS signal, such as, for example, code-division multiple access (CDMA) signal. CDMA digital communication takes place using spread spectrum techniques that broadcast signals having wide bandwidths, such as, for example, 1.2288 megahertz (MHz) bandwidths. Generally, a channel having a bandwidth that is substantially smaller than a wideband channel, it is referred to as a narrowband channel. For example, an RF power generated by an inter-modulation product, a harmonic signal, etc., may be generally referred to as a narrowband signal.
The switching station 18 is generally responsible for coordinating the activities of the base stations 14, 16 to ensure that the mobile units 12, 13A, 13B, 13C, and 13D are constantly in communication with the base station 14, 16 or with some other base stations that are geographically dispersed. For example, the switching station 18 may coordinate communication handoffs of the mobile unit 12 between the base stations 14 and another analog base station as the mobile unit 12 roams between geographical areas that are covered by the two base stations.
One particular problem that may arise in the telecommunication system 10 is when the mobile unit 12 or the base station 14, each of which communicates using narrowband channels, interferes with the ability of the base station 16 to receive and process wideband digital signals from the digital mobile units 13A, 13B, 13C, and 13D. In such a situation, the narrowband signal transmitted from the mobile unit 12 or the base station 14 may interfere with the ability of the base station 16 to properly receive wideband communication signals.
FIG. 2 illustrates a typical frequency bandwidth of a telecommunication system using DSSS system. In particular FIG. 2 illustrates a frequency spectrum 50 of a 1.288 MHz DSSS system used by the digital mobile units 13A, 13B, 13C, and 13D to communicate with the base station 16, and a 200 kHz frequency spectrum 52 used by the module unit 12 using a narrowband digital communication system. It would be obvious to one of ordinary skill in the art that at times, the digital signal shown in 52 may interfere with the frequency spectrum 50.
If a filter were to be designed to remove the interference caused by the 200 kHz analog signal 52 from the DSSS signal 50, the transfer function of such a filter may be given by:
                              Φ          ⁡                      (            f            )                          =                                                                          S                ⁡                                  (                  f                  )                                                                    2                                                                                                S                  ⁡                                      (                    f                    )                                                                              2                        +                                                                            N                  ⁡                                      (                    f                    )                                                                              2                                                          (        1        )            
Wherein |S(f)|2 is the power spectral density (PSD) of the desired signal and |N(f)|2 is an estimate the PSD of the interference (noise) signal. If the nature of the interfering signal (noise term N) is assumed to be that given by the analog interference signal 52, the shape of the filter may be given as shown by a notch frequency spectrum 54 illustrated in FIG. 2. However, the frequency at which the notch filter should be placed is unknown in general. For effective application of the notch filter having the notch frequency spectrum 54, it is necessary to determine the location of the interference signal 52.